1. Field of the Invention
The present invention generally relates to a process of trimming a photoresist mask on a semiconductor substrate.
2. Description of the Related Art
To increase operational speed of devices (e.g., transistors, capacitors, and the like) in integrated microelectronic circuits, the device features have become ever smaller. The minimal dimensions of features of such devices are commonly called in the art, critical dimensions, or CDs. The CDs generally include the minimal widths of the features, such as lines, columns, openings, spaces between the lines, and the like.
One method of fabricating such features comprises forming a patterned mask (e.g., photoresist mask) on the material layer beneath such a mask (i.e., underlying layer) and then etching the material layer using the patterned mask as an etch mask.
The patterned masks are conventionally fabricated using a lithographic process when a pattern of the feature to be formed is optically transferred into a layer of photoresist. Then, the photoresist is developed and unexposed portions of the photoresist are removed, while the remaining photoresist forms a patterned mask.
An etch mask generally is, in a plan view, a replica of the feature to be formed (i.e., etched) in the underlying layer. As such, the etch mask comprises elements having same critical dimensions as the feature to be formed. Optical limitations of the lithographic process may not allow transferring a dimensionally accurate image of a feature into the photoresist layer when a CD of the element is smaller than optical resolution of the lithographic process.
To overcome limitations of the lithographic process, the photoresist mask may be fabricated using a two-step process. During a first step, the lithographic process is used to form the mask having elements with dimensions that are proportionally greater (i.e., “scaled up”) than the dimensions of the features to be formed. During a second step, such “scaled-up” elements are trimmed (i.e., isotropically etched) to the pre-determined dimensions. The trimmed photoresist mask is then used as an etch mask during etching the underlying material layer or layers.
One problem in trimming such a photoresist mask is the occurrence of critical dimension (CD) microloading, which is a measure of variation in critical dimensions between dense and isolated regions of the substrate after photoresist trimming. The dense regions have a high pattern density of the features and the isolated regions have a low pattern density of the features. Conventional photoresist trimming processes often result in significant CD trimming microloading with the isolated regions being trimmed at much faster rates than dense regions.
Therefore, there is a need in the art for an improved method for controlling photoresist trimming process to reduce microloading effect during fabrication of semiconductor devices in a semiconductor substrate processing system.